We will study the effects of ionizing radiation at low dose rate on human diploid fibroblasts from patients with known or proposed DNA repair defects. The responses of these cells (ataxia-telangiectasia, xeroderma pigmentosum and Fanconi's anemia) will be compared to the response of normal human fibroblasts. Radiation damage-time interactions will be studied as these are expressed by survival, i.e., capacity to divide without limit and repair capacity. Two culture conditions will be used: 1) exponential and 2) plateau phase of growth.